1. Field of the Invention
This invention relates to fuel element spacer assemblies for nuclear reactors and, more particularly, to an apparatus for limiting spacer assembly movement.
2. Summary of the Prior Art
In water cooled heterogenous nuclear reactors, the reactor core in which the fission chain is sustained generally contains a multiplicity of fuel assemblies that are identical in mechanical construction and mechanically interchangeable in any core fuel assembly location. Each fuel assembly is designed to maintain its structural integrity during reactor heatup, cooldown, shutdown and power operation including the most adverse set of operating conditions expected throughout its lifetime. Design considerations for reactor operation typically include the combined effects of flow induced vibration, temperature gradients, and seismic disturbances under both steady state and transient conditions.
Each fuel assembly typically contains a plurality of thin elongated fuel elements, a number of spacer grid assemblies, guide tubes, an instrumentation tube, and end fittings.
The fuel elements often used in current applications are known as fuel rods and comprise cylindrical UO.sub.2 fuel pellets, stacked end to end within thin walled tubes, having spring loaded plenums at each end of the tube, and sealed with end caps or plugs. The fuel rod cladding is made from a material, such as a zirconium alloy, which has good neutron economy, i.e., a low capture cross section. Each fuel rod is capable of transferring fission generated heat to a circulating coolant while containing the fission products and the fuel material. The fuel rods must withstand the effects of thermal cycling, fission gas release to the rod plenum, fuel densification, cracking and ratcheting of the fuel pellets, pellet-clad mechanical interactions, and corrosion or radiation induced changes in the mechanical properties of the cladding.
Depending upon the position of a fuel assembly within the core, a number of elongated cylindrical guide tubes, arranged in parallel with fuel rods, are used variously to provide continuous sheath guidance for control rods, axial power shaping rods, burnable poison rods, or orifice rods. The guide tubes, which usually have a larger diameter than the fuel rods are provided with sufficient internal clearance to permit coolant flow therethrough to limit the operating temperatures of the absorber materials which may be inserted therein, and to permit rod insertion and withdrawal motions within the guide tubes as required during all phases of reactor operation.
The fuel rods and guide tubes are supported in a square array at intervals along their lengths by spacer assemblies that maintain the lateral spacing between these components. The spacer assemblies are generally composed of a multiplicity of slotted rectangular grid plates arranged to intersect and interlock in an egg-crate fashion to form cells through which the fuel rods and guide tubes extend. Illustratively, the grid plates may be of the type such as described in U.S. Pat. No. 3,665,586 by F. S. Jabsen and assigned to The Babcock & Wilcox Company which have indentations that laterally extend essentially perpendicular to the longitudinal axes of the fuel rods into those cells which have fuel rods for engagement and support of the fuel rods.
The spacer assemblies maintain a necessarily precise spacing between fuel rods in order to avoid neutron flux peaks and regions of abnormally high temperature (hot spots) where burnout, i.e., severe local damage to the fuel rods, could occur. The spacer assemblies assure the mechanical stability that is essential to preclude the distortions which may be otherwise caused by flow induced vibrations, pressure differences, and thermal stress. However, the number of spacer assemblies used to provide for fuel element support within the core must be balanced with the countervailing need to minimize this structural material which contributes to the parasitic absorption of neutrons. Coolant circulating through the spacing between the fuel rods must be sufficient to remove the fission generated heat. The spacing between fuel rods is quite restricted, for example, on the order of 0.15 inches. Thus, incipient deviations from design spacing dimensions, cladding eccentricities, surface roughness and warping markedly affect distribution of the coolant flow about the fuel rods and may lead to localized flow restrictions in the coolant channels, causing a rise in fuel rod surface temperature at the points of restrictions. Hence, it is extremely important that the spacer assemblies remain fixed throughout a fuel assembly's life.
A number of means have been employed in the prior art to limit spacer assembly movement. In the past, for example, the guide tube wall diameter has been swaged out to engage the surfaces of surrounding sleeves attached adjacent each end of the grid plates of a spacer assembly. This has the effect, however, of distorting the guide tubes and spacer grids due to the material drawing mechanism of the swaging operation, thereby inducing (1) variations in the parallelism of the spacer assemblies and (2) fuel element bow. Attempts to restrain spacer assembly movement by welding the guide tubes directly to the grid plates also have made the maintenance of parallelism difficult.
An arrangement for restraining and retaining the spacer assemblies in place without tending to induce variations in the relative parallelism of several spacer assemblies within a fuel assembly and without causing fuel rod bow is highly desirable.